Add support for 8 bit thermal camera image format

[iche033]

Summary

The existing thermal camera implementation produces image output in 16 bit format and that's being hard coded in various places including the shaders. This PR makes the implementation slightly more generic to support generating 8 bit data.

There are a few workarounds that had to be done so that this change does not break ABI. For example, the new thermal image event's function callback is hard coded to uint16_t and that can not be changed so unfortunately I had to workaround this by packing 8 bit data into 16 bit array.

Checklist

• Signed all commits for DCO
• Added tests
• Updated documentation (as needed)
• Updated migration guide (as needed)
• codecheck passed (See contributing)
• All tests passed (See test coverage)

Note to maintainers: Remember to use Squash-Merge

---

[codecov]

Codecov Report

Merging #235 (8a5bf29) into ign-rendering4 (38d1ec0) will increase coverage by 0.12%.
The diff coverage is 100.00%.

@@                Coverage Diff                 @@
##           ign-rendering4     #235      +/-   ##
==================================================
+ Coverage           52.75%   52.87%   +0.12%     
==================================================
  Files                 143      143              
  Lines               13493    13522      +29     
==================================================
+ Hits                 7118     7150      +32     
+ Misses               6375     6372       -3     

Impacted Files	Coverage Δ
ogre2/src/Ogre2ThermalCamera.cc	90.70% <100.00%> (+0.82%)	⬆️
src/PixelFormat.cc	73.33% <0.00%> (+10.00%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 38d1ec0...8a5bf29. Read the comment docs.

[adlarkin]

All comments have been addressed, and all checks have passed - LGTM!